The present invention relates to a communication system, and, more particularly, to a system for communication between stored program controls.
Stored program controls have been decentralized to increase their efficiency. Small, locally distributed units take care of specific tasks for the control of industrial processes. If the solving of problems or the status of individual stored program controls is of importance for other stored program controls, these stored program controls must be able to communicate with one another by exchanging data. The requirements for a communication system will differ from case to case. For example, in office communication, maximum speed of data transmission is not of particular importance, while the control of industrial processes usually requires data processing in real time.
The coupling of stored program controls with one another can take place by parallel or serial coupling or over bus systems. During the time in which the coupled stored program controls exchange data with one another, these stored program controls, including their respective bus system, are blocked for other tasks. In data transmission, measures must be taken which increase the time required for the exchange of data. Such measures are, for example, the recognition of data losses, whereupon data must be retransmitted, and the recognition of a failure in the stored program controls. Invalid data must have a defined state, an interrupt treatment of important data must be provided, and test functions must exist which furnish to the user the state of all data elements. Furthermore, no so-called "deadlock" situations should be possible, in which data exchange does not occur due to undefined data states.
There is a need for a communication method which, taking into consideration all the aforementioned requirements, makes possible a very rapid data exchange of coupled data processing installations.